Prior art systems for achieving composting of solid waste and sewage sludge typically employ one or more multi-stage digesters in which material being treated undergoes staged microbial decomposition. The conventional digester comprises a tube-like structure divided into two or more compartments or stages. During material processing the tube is rotated while air is circulated through the digester at controlled rates under predetermined conditions in a flow direction counter to the material flow. The climate in each stage is maintained to achieve the optimum development of the type and species of microorganism predominant in that stage. Spent air is vented from the digester to the atmosphere as needed to maintain an optimum climate in each of the operating stages. Temperatures are kept below 150 degrees fahrenheit to insure the maximum rate of composting. Moisture content of the material under treatment usually does not require adjustment during this phase of the operation, but if the material becomes too dry for composting, the digester vessel is equipped with a water manifold which allows the addition of moisture to the mass undergoing treatment. Since this phase of the composting is conducted entirely within an enclosed vessel, the only source of odor production is the exhaust air. Typical of such prior art systems and methodology of operation are those set out and described in U.S. Pat. Nos. 3,245,759 and 3,138,447, the teachings of which are hereby incorporated by reference.
The method and apparatus for manufacture of compost described in those patents is designed to produce aerobic decomposition of organic waste materials by maintaining within the apparatus in which the method is carried out, conditions suitable for optimum propagation of the different types of aerobic bacteria on which such decomposition depends. The apparatus as noted above comprises a digester in the form of a cylindrical drum mounted for rotation on an axis which is slightly declined towards the discharge end relative to the horizontal. The interior of the digester is divided into a series of compartments or chambers by a plurality of transverse partitions spaced along the axis of rotation. Each partition is provided with transfer buckets which are selectively opened and which, when opened transfer material from compartment to compartment from the higher to the lower end of the drum, the raw waste organic material being fed into the digester at the higher end and the finished product being withdrawn at the lower end.
Environmental, regulatory and recycling forces have led to a refocusing of disposal options. Landfilling, which currently accounts for approximately 78% of municipal solid waste disposal, is in severe jeopardy. The number of landfills has decreased from 20,000 in 1978 to 6,000 in 1988 and is estimated to decrease to 2,100 by the year 2000. In addition, incineration, which accounts for approximately 11% of municipal solid waste disposal has come under intense public scrutiny. Public opposition has been responsible for the curtailment of a significant number of proposed incineration projects.
The co-composting technology to which the present invention has application embodies a fermentation reactor which is employed to accelerate the microbial conversion of solid waste and sewage sludge into a high quality compost. The process has the ability to compost municipal solid waste and sewage together hence the term co-composting, thus addressing the two principle waste management problems communities will face in the next few decades. Unlike incineration and landfilling, which both generate a number of toxic by-products including harmful emissions of gases, heavy-metal laden ash and leachate, the co-composting process to which this invention relates produces no toxic by-products. Additionally, it is a non-grind, non-shred, in-vessel aerobic process that produces a high quality compost while simultaneously sanitizing non-compostable residual tailings. The process results in municipal solid waste volume reductions as high as 85% while simultaneously providing a solution to the sewage sludge problem. This is extremely important in light of the fact that, based upon current projections, municipal solid waste generation will exceed landfill capacity by approximately 250% by the year 2000.